The rear derailleur freewheel/cassette and chainwheels are probably the first to be changed on a bike.
For Treks, the SR date markings are especially important.
Virtually all of the Treks not equipped with Campagnolo or Shimano Dura-Ace components, started life with some grade of SR seatpost and may also have SR stems, bars or cranks.
As the wear on a vintage bike increases, the greater the likelihood components are not original, either through replacement of worn parts or through component swaps.
Swaps can take place early in the life of a bike as the proud new owner upgrades to new or used components of higher quality.
)Most early Treks (1976 through about 1980) were sold as framesets.
The components were added by the local bike shop or by the buyer. Components also could be swapped from an existing ride to the new frameset.
Unfortunately, many of these are coded, and require some additional knowledge to understand the code.
If you know of other components that are marked or coded that can be added to this list, please let me know.: The information on this page is copyrighted.
This makes dating the components an interesting archeological investigation, but one not necessarily related to the date of the bike. Trek owner Larry Osborn made this observation, and suggested this as a supplementary way of dating a Trek (and other bikes as well).
Fueled by this first realization, and with the help of other bike folks, Larry and I have sorted out other codes (a project still ongoing).
Feel free to quote small parts of the information on this page, but copyright law requires be listed as the source of the information.